[Greenbuilding] Drying House in Humid Season

[nick pine]

"conservation architect" <elitalking at rockbridge.net> wrote:

> It has been noted on this list the problems associated with night flushing 
> in humid climates such as here in Virginia in the summer.  I have engaged 
> in this practice for many years, because it brings down the temperature of 
> the house to low 70s most nights and keeps the house below 80 most days 
> when outside air temp is in 90s.  My house has most of a high standard 
> retrofit continuous foam thermal envelope.  I have an HRV.  I choose not 
> to get ERV because I did not plan to install active dehumidification.   To 
> do so,  I tolerate sometimes 80% relative humidity in mid-summer.  I am 
> not willing to accept that one must have refrigerant driven 
> dehumidification to achieve healthy conditions.  However, for my clients 
> that expect a full comfort standard, this is what I must specify.  The 
> people who choose to live without air conditioners acclimate to higher 
> amounts of ambient humidity.  I went to primary school in Central Florida 
> with no air conditioners.  None of us complained because ac was not yet 
> expected in 1960s.

> The last few years I have been observing more carefully relative humidity, 
> where previous years I only observed temperature.  I have observed that a 
> house as a whole goes through a cycle of steadily becoming more humid 
> through the summer and peak in early fall and then go into a drying mode 
> when heating air.  The drying peaks in early spring (end of heating 
> season).  It is my observation that the house as a whole acts somewhat 
> like a desiccant in that it adsorbs humidity in the early part of summer 
> as the conditions deliver humid air to the space.

IIRC, concrete absorbs about 1% of its weight in water as the RH of the 
surrounding air rises from 40 to 60%, and unpainted wood and paper and 
fabrics absorb more... 
http://www.conservationphysics.org/isothrm/isothrm.php

>The house's initial dryness in early spring dampens the air relative 
>humidity in early part of humid season.  However, steadily, the house's 
>capacity to adsorb humidity is maxed out and air relative humidity rises 
>farther.  This creates the slow drying conditions of contents that cause 
>the problems associated with humidity.  This past year, I have added 
>absolute humidity to my observations with the use of a psychometric chart 
>using temperature and relative humidity.

You can also estimate the absolute humidity with a Clausius-Clapeyron 
approximation: vapor pressure Pa = RH/100e^(17./863-9621/(T+460)) "Hg, with 
RH in % and T in degrees F, and humidity ratio w = 0.62198/(29.921/Pa-1) 
pounds of water per pound of dry air. For example, 70 F air at 50% RH has Ph 
= 0.374 "Hg and wh = 0.00788 pounds of water per pound of dry air... 50 F 
air at 60% RH has Po = 0.220 "Hg and wo = 0.00461 pounds of water per pound 
of dry air. Air weighs about 0.075 lb/ft^3, so replacing 2000 cfm 70 F house 
air at 50% RH with 50 F outdoor air at 60% removes 2000x60m/hx0.075(wh-wo) = 
29 pounds per hour (vs 29 pints per day with a compressor) of water vapor.

>This also gives dew point.  Absolute humidity and dew point are directly 
>proportional.  I observed that on a hot fair weather day (no 
>precipitation), the absolute humidity was the same inside and out.  If 
>inside was 75F-80%RH and outside 95F-45%RH, the absolute humidity and dew 
>point were approximately the same.  Although the hotter air would be more 
>affective at drying and avoiding humidity problems.

I believe that Smart Vents do this sort of calculation... 
http://www.smartvent.net/

> This was a particularly humid summer. Early summer was rainy and cooler 
> than normal.  This resulted in elevated humidity.  One day I observed 
> 71F-88%RH in the house.  This was alarming, so I decided to heat the house 
> above comfort in order to dry it out.  Like a clothes dryer, I was 
> elevating the temperature to lower the relative humidity and increase 
> evaporation.  That day was in mid 80's, so was not an extremely hot day. 
> Since I was not going to occupy the house I turned the HRV off to increase 
> my temperature to avoid delivering new humid air. I lit the direct air 
> delivered wood stove to a mid-winter full size fire.  I got the house to 
> 104F.

Wow.

>I found that the absolute humidity went way up over what would be expected 
>if no moisture was added to the air.  This was the moisture being 
>evaporated from house contents inside thermal envelope.  Since the house 
>was closed up, I would assume minimal infiltration.  After a few hours of 
>this, I opened up the house to vent out the heat and moisture.  Returning 
>to night flushing I was able to return house to low 70s and 10% lower RH by 
>morning.  This returned my house to an earlier time in the humid season.

"Smart ventilation" might do this more often with less energy, in times of 
low outdoor absolute humidity.

> I would propose that intermittently heating a house that has humidity 
> build up is a way to manage the humidity.  The mold is dried and must 
> start all over. Yes, control humidity at the source where possible. 
> However, when the ambient conditions are humid that strategy is 
> incomplete.   This heating is a is appropriate where the house has low 
> mass, allowing the temperature to be easily raised and returned to normal. 
> I would also propose that we take steps to build up the desiccant capacity 
> of the house such that fewer cycles such as this would be needed.  Because 
> we are taking away the humidity in the humid seasons, it may result in the 
> house being dryer in the early heating season.  A strategy of 
> humidification might be considered.

HUmidification?

> I am aware that there is risk if air is flowing through the thermal 
> envelope and cooling to dew point as a result of higher indoor humidity. 
> However, if a well designed and built thermal envelope with a thick layer 
> of foam that is high enough proportion of the total R value such that 
> humidified air that would penetrate to the warm side will be above dew 
> point in the expected coldest conditions, this could work.  With those 
> risk managed, humidification is much lower energy consumption and 
> environmental risk than the refrigerant driven dehumidification.
>
> Perhaps leaving the windows open during the day and not pulling cold moist 
> air into the house at night is another way to reduce humidity risk. 
> Though the absolute humidity would be about the same, the higher relative 
> humidity resulting from the higher temperature would be less vulnerable to 
> humidity problems.  Although, I contend it is less comfortable.

Smart Vents have fairly small fans, but I think they also have an outlet 
that can be used to run a larger fan.

Nick